1. Field of the Invention
The present invention relates generally to circuits for recovering stereo signals in radio receivers. More specifically, the present invention relates to techniques for recovering a pilot tone and left and right channel FM stereo signals from demodulated FM signals.
2. Description of the Related Art
Many radio transmitters radiate a transmission signal including stereo signals, such as FM stereo signals. To recover stereo signal information from the transmitted FM stereo signals, a radio receiver will demodulate and isolate the FM stereo signal information within the FM stereo signals. The transmitted FM stereo signals typically include left-minus-right (Lxe2x88x92R) signal information, left-plus-right (L+R) signal information and a pilot tone. For example, once modulated, the left-minus-right (Lxe2x88x92R) signal information may be centered at 38 kHz. The left-plus-right (L+R) signal information may be centered at baseband or DC. The pilot tone may be located at 19 kHz. Typically, the 19 kHz pilot tone is used to demodulate the stereo signal information into its constituent left (L) and right (R) components. Prior stereo decoders, however, have suffered from various inefficiencies and inaccuracies in recovering the 19 kHz pilot tone and, thereby, in recovering the stereo signal information embedded within the FM stereo signal.
According to the present invention, stereo recovery circuitry for a radio receiver provides increased accuracy and efficiency in recovering stereo signal information from transmitted stereo signals. The stereo decoder includes a digitally controlled oscillator that recovers a pilot tone signal from transmitted stereo signal information. By processing demodulated stereo signals on the digital side and digitally controlling the oscillator, the stereo decoder has increased efficiency and accuracy. In one embodiment, the oscillator may be a phase-locked-loop having a loop filter and an amplitude controlled tunable resonator. Additional circuitry is disclosed for utilizing the pilot tone signal to recover left and right channel signal information from the demodulated stereo signals.